<CSG (Closed Subscriber Group) Cell>
In the 3GPP, a base station managed by the CSG (Closed Subscriber Group) (hereinafter, referred to as “CSG base station”) is studied which provides a cell that is called a CSG cell, and that issues an access permission to a specific user. As a CSG base station, for example, a Home eNodeB (HeNB or a Home NodeB: HNB) may be contemplated.
FIG. 19 shows the architecture configuration of a CSG base station system. As shown in FIG. 19, CSG cells are constructed in a cell (macro cell) constructed by a usual macro base station (eNB). Unlike a macro base station, a CSG base station is provided with a function that can restrict an accessible terminal (User Equipment: hereinafter referred to as “UE”). Therefore, a UE can be connected only to a CSG cell having an access permission. Even when a CSG cell having a very high quality is detected, a UE cannot be connected to the CSG cell without an access permission, except in an emergency.
When a UE is to check an access permission of a CSG cell, the UE must check cell identification information contained in notification information sent from a CSG base station. Each CSG cell has an ID called “CSG ID” in a unit of group, and gives an access permission to a UE in a unit of CSG ID. The UE collates “CSG ID list” (access permission list: called “CSG white list”) of accessible CSG cells which is notified by a network, with a CSG ID which is cell identification information contained in notification information sent from a detected CSG base station. When the CSG ID of the detected CSG cell is contained in the CSG white list, the UE determines that access is enabled. In the following description, a CSG ID is referred to as “access permission information”.
<PCI Confusion>
All cells have cell identification information which is called a Physical Cell ID (PCI). With respect to CSG cells, there is a case where, as shown in FIG. 20, a plurality of CSG cells having the same PCI exist in the vicinity of a serving cell provided by a serving base station to which the UE is currently connected.
The UE measures the reception quality of a signal sent from a peripheral cell, and is triggered when the measured reception quality of the serving cell or peripheral cell satisfies conditions. This is called event trigger. When the event trigger is activated, the UE notifies the serving cell of the PCI of a cell having a high reception quality by using a measurement report message. Based on the information of the reception quality contained in the measurement report message, the serving base station determines the handover destination of the UE, and transmits a handover request (HO request) message to the base station of the handover destination. At this time, in the case where a plurality of CSG cells having the same PCI exist in the vicinity of the serving base station, there is a possibility that the serving base station may transmit a handover request (HO request) message to a base station of an erroneous CSG cell. This is called “PCI confusion”. FIG. 20 shows a PCI confusion.
<Operation Before Handover>
FIG. 21 shows the procedure for a measurement report message which is an operation before a handover, the message containing a CSG cell. A UE holds past information (fingerprint) of cells to which it has been connected in the past. As shown in FIG. 21, when approximation to a CSG cell to which an access permission has been issued is detected based on the past information, the UE notifies the serving base station of one of frequencies of the CSG cell, by means of a proximity indication message. In the case where the frequency notified by the proximity indication message is not contained in the measurement configuration information, the serving base station notifies the UE of the configuration information by means of a measurement configuration message. Therefore, the UE can immediately detect and measure a CSG cell having an access permission.
In order to prevent a PCI confusion from occurring, a UE detects a Cell Global ID (CGI) which is a unique cell identifier contained in notification information sent from a CSG base station, and notifies a serving base station of the CGI by means of the measurement report message, so that the serving base station identifies a CSG cell. However, it takes time before the UE receives the notification information from the CSG base station. Therefore, the UE first notifies the serving base station of the ID (PCI) of a cell having a high reception quality and the measured reception quality, by means of a first measurement report message. The serving base station selects one cell which is a handover destination candidate, from cells contained in the received first measurement report message. In the case where the selected cell is a CSG cell, and, as a result of a check of proximity information, is a cell corresponding to a frequency notified by the proximity indication message, in order to instruct the UE to detect the CGI of the CSG cell, the serving base station notifies the UE of a measurement configuration message containing configuration information which is necessary for detecting the PCI and CGI of the CSG cell. Based on the measurement configuration message, the UE receives notification information of the instructed CSG cell to acquire System Information (SI) containing CSG cell information such as the CGI. This process is called “SI reading”. By using access permission information (CSG ID) contained in the CSG cell information, the UE checks the existence of an access permission. The UE notifies the serving base station of the cell information of the CSG cell and the result of the access check after the access permission checking process, by means of a second measurement report message. This process is called “SI report”.